Methods of obtaining contours
CE 221 ‐ Higher Surveying
By estimationBy estimation
• Suitable for small scale maps where the ground p gform is not too irregular
• The process is fast and produce reasonable results when done carefullyresults when done carefully
• Steps:– Make a careful study of the position of the contourMake a careful study of the position of the contour points and try to visualize the changes in slope of the terrainDraw the contours which appear to be most definitely– Draw the contours which appear to be most definitely located by the plotted points; index contours are usually drawn first which will act as guides for the intermediate contoursintermediate contours
3Prepared by: Andre‐Paul C. Ampong
Rubber band methodRubber band method
The rubber band should have sufficient width so that graduations at equal intervals can be marked on it to form some kind of an elastic scale. 4Prepared by: Andre‐Paul C. Ampong
Rubber band methodRubber band method
The rubber band is stretched between two plotted points so that p pthese points fall at scale divisions corresponding to their elevations 5Prepared by: Andre‐Paul C. Ampong
Variable scalesVariable scales
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Analytical methodAnalytical method
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Analytical methodAnalytical method• By similar triangles:
Where:DEH
dehm =
Where:A & B are established points on the
ground of known elevationM is a point on a contour crossing p g
the line between A and Bhm is the horizontal map distance
from A to M (cm)de is the difference in elevationde is the difference in elevation
between M and A (m)DE is the difference in elevation
between A and B (m)H is the horizontal map distance
between A and B (cm)8Prepared by: Andre‐Paul C. Ampong
Triangle and scale methodTriangle and scale method
9Prepared by: Andre‐Paul C. Ampong
Triangle and scale methodTriangle and scale method
Elev = 296.9m
El 245 5Elev = 245.5m
GiGiven:Elevation of point “a” = 245.5 mElevation of point “b” = 296 9 mElevation of point b = 296.9 mContour Interval = 20.0 m
10Prepared by: Andre‐Paul C. Ampong
Triangle and scale methodTriangle and scale method
Elev = 296.9m
El 245 5Elev = 245.5m
11Prepared by: Andre‐Paul C. Ampong
Triangle and scale methodTriangle and scale method
Elev = 296.9m
El 245 5Elev = 245.5m
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Triangle and scale methodTriangle and scale method
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Template methodTemplate method
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Template methodTemplate method
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Template methodTemplate method
• Sample problemSample problem
i d i d i l b i dIt is desired to interpolate between points X and Y the position of the following contours: 161, 162 163 164 d 165 Th l i f X d162, 163, 164 and 165. The elevation of X and Y are respectively 160.9 m and 165.4 m.
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Template methodTemplate method
Y (Elev = 165 4 m)Y (Elev = 165.4 m)5.4
165 m
163 m
164 m
162 m
X (Elev = 160.9 m)0.9
161 m
17Prepared by: Andre‐Paul C. Ampong
Template methodTemplate method
• Sample ProblemSample Problem
l d d hAssume two plotted ponts A and B have elevations of 117.5 m and 125.8 m,
i l I i i d lrespectively. It is required to locate contours 120 and 125 using a template of converging lilines.
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Template methodTemplate method
A (Elev = 117.5 m)
B (Elev = 125.8 m)
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Template methodTemplate method
A (Elev = 117.5 m)
01
B (Elev = 125.8 m)1
23
45
677
8
20Prepared by: Andre‐Paul C. Ampong
Template methodTemplate method
A (Elev = 117.5 m)
01
B (Elev = 125.8 m)
12
34
56
7 88
21Prepared by: Andre‐Paul C. Ampong